Help needed - Fungus in garden

We have a wooden border made from thick wooden sleepers around the bottom of our raised grassed area. Over the last year, a fungus has grown in one area around the bottom of the grassed area and onto the wood. It is now expanding and eating/rotting the wood.

I don't have a clue about anything like this so was wondering whether anyone can identify the fungus and tell me what I need to do to get rid of it.

There's no cure as such. Pre-treated wood will last longer but will eventually succumb to one of the thousands of types of fungi (only a laboratory could definitely ID which one.) However, it could last another 5 years before falling apart completely. The only real solution is replacement - pre-treated wood can last 15 years but if in permanent contact with wet soil it can be much less. That's why railway sleepers used to be soaked in boiling tar, which makes them last for decades. Unfortunately, it was found to be rather toxic and carcinogenic, so you can't generally find them for domestic use any longer.

I agree 4 years is a bit of a short lifetime for a new one, Gillibobs. It might be that particular piece of timber was already infected. Do you know the type of wood used, as Welshonion said? Oak is one of the longest lasting ones.

I haven't got a clue, we didn't specify any type of wood, or even to use wood in particular that's for sure, it was just the design he came up with and we were happy with how it looked. Our garden is sloping though and any excess water would drain to that particular area so I don't know if that helped to accelerate it.